Pericentre distance ($q$)

The pericentre distance is the minimum separation from the attracting centre on an orbit, measured to the periapsis. For an elliptic orbit of semi-major axis $a$ and orbital eccentricity $e$,For example, $a=10$ length units and $e=0.9$ give $q=1$ unit even though the orbit extends to apocentre distance $Q=19$.

This distinction matters for close encounters, tidal effects and heating: a large semi-major axis does not guarantee a large closest approach. The distance is measured from the centre, rather than as altitude above a physical surface. A parabolic orbit has a finite pericentre distance despite lacking a finite elliptic semi-major axis.